A chelating agent is a compound containing donor atoms that can combine by coordinate bonding with a single metal ion to form a cyclic structure called a chelation complex or, simply, a chelate. Because the donor atoms are connected intramolecularly by chains of other atoms, a chelate ring is formed for each donor atom after the first which coordinates with the metal. Each ring gives the appearance of a metal atom being held in a pincer formed by other atoms. The technological importance of chelation is based on the almost universal presence of metal ions of one kind or another. They are present either naturally or, in certain instances by intentional addition. Chelating agents provide a means of manipulating and controlling metal ions by forming complexes that usually have properties that are markedly different from those of the original ions or the chelants. These properties may serve to reduce undesirable effects of metal ions as in sequestration, or to create desirable effects as in metal buffering and solubilization.
The structural essentials of a chelate are coordinate bonds between a metal ion and two or more atoms in the molecule of the chelating agent. The coordinating atoms of the chelating agent are electron donors and the metal ion is an electron acceptor. When coordinate bond formation occurs between the metal and two donor atoms, the atoms of the ligand that connect the donor atoms complete the ring that gives the structure its chelate character.
Commerically useful chelating agents include ethylene-diamine tetraacetic acid (EDTA) and citric acid. Other compounds known to be useful as chelating agents include ethylenediamine-N,N'-diacetic acid, alanine-N,N'-diacetic acid, anthranil-N,N'-diacetic acid and 1,2-dihydroxyanthraquinone-3-methylamine-N,N'-diacetic acid.